The invention relates to a method of acquiring fragment ion spectra in Kingdon ion traps which have a potential well for harmonic oscillations of the ions in the longitudinal direction and in which the ions can oscillate radially in a plane between two or more inner electrodes. Kingdon ion traps are electrostatic ion traps in which the ions orbit with a predefined kinetic energy around an inner electrode arrangement or oscillate through an inner electrode arrangement. The inner electrode arrangement is enclosed by an outer housing electrode arrangement kept at a potential which the ions cannot reach. The outer and the inner electrode arrangements can be shaped in such a way that, firstly, the motions of the ions in a longitudinal direction of the Kingdon ion trap are completely decoupled from the motions in a radial direction, and secondly, a potential well is generated in the longitudinal direction, in which the ions can oscillate harmonically, independent of their motion in the radial direction. For longer storage times, a Kingdon ion trap must be operated under ultrahigh vacuum because, otherwise, the ions lose their kinetic energy by collisions with the residual gas and finally impinge on the inner electrode arrangement.
If radially orbiting or radially oscillating ions being confined in the longitudinal direction in a narrow slice are excited to coherent harmonic oscillations in longitudinal direction in the potential well, the ions of different charge-related masses separate because they oscillate at different frequencies. The frequencies are inversely proportional to the square root √(m/z) of the charge-related mass m/z. With suitable detection electrodes, such as an outer electrode arrangement consisting of two symmetric halve-shells split vertically to the longitudinal direction, the image currents of these oscillations can be measured at these half-shells as temporal transient signals. A Fourier analysis delivers the spectrum of the ion oscillations in longitudinal direction from this image current transient, and a mass spectrum can be obtained from the frequency spectrum. As with other Fourier transform mass spectrometers, a very high mass resolution R=m/Δm can be achieved, Δm being the width of the mass signal of mass m at half height. The precondition is that the inner and outer electrode arrangements are very precisely manufactured, because the harmonicity of the potential well and the independence of radial and longitudinal oscillations depend on their shape.
The expression “Kingdon ion trap mass spectrometer” should refer to a mass spectrometer including an Kingdon ion trap, in which (a) the oscillations in radial and longitudinal direction are decoupled, (b) the longitudinal potential well allows for harmonic oscillations of the ions in longitudinal direction, and (c) there are means for measuring the oscillations in longitudinal direction by their image currents.
The advantage of Kingdon ion trap mass spectrometers compared to ion cyclotron resonance mass spectrometers (ICR-MS) with a similarly high mass resolution R is that no superconducting magnet is required to store the ions and so the technical set-up is less complex and costly. Moreover, the decrease in resolution R in Kingdon ion trap mass spectrometers is only inversely proportional to the square root √(m/z) of the mass of the ions, whereas the decrease in resolution R in ICR-MS is inversely proportional to the mass m/z itself; this means the resolution falls off much more rapidly towards higher masses in ICR-MS.
U.S. Pat. No. 5,886,346 (A. A. Makarov, 1995) elucidates the basics of a Kingdon ion trap mass spectrometer which later was introduced onto the market by Thermo-Fischer Scientific GmbH Bremen under the name Orbitrap™. The Orbitrap™ consists of a single spindle-shaped inner electrode and a coaxial outer electrode, the outer electrode having an ion-repelling electric potential and the inner electrode an ion-attracting electric potential. With the aid of a complicated ion introduction system, the ions are injected as ion packets tangentially to the inner electrode, and move in a hyperlogarithmic electric potential. The kinetic injection energy of the ions is set so that the attractive forces and the centrifugal forces balance each other out, and the ions therefore move on virtually circular trajectories. In the longitudinal direction of the electrode axis, the electric potential of the Orbitrap™ has a potential well, in which the ion packets can execute harmonic oscillations. The harmonically oscillating ion packets induce image currents in the half-shells of the centrally split outer electrode arrangement and these currents are measured as a function of time. The mass resolution of an Orbitrap™ is currently about R=50,000 at m/z=1,000 daltons, and even higher for good instruments. The electrodes must be manufactured to a very high degree of mechanical precision. In addition, the injection of the ions is critical because the kinetic energy of the ions on injection must only vary within a small tolerance range.
The U.S. patent application Ser. No. 12/098,646 (C. Köster, corresponding to DE 10 2007 024 858.1) describes a further type of Kingdon ion trap with several embodiments which feature several inner electrodes in different arrangements. Here too, the inner electrodes and the outer enclosing electrodes can be precisely formed in such a way that the longitudinal motion is completely decoupled from the radial motion and a potential well for generating harmonic oscillation is created in the longitudinal direction. The patent application contains mathematical expressions for equipotential surfaces inside such Kingdon ion traps, and these expressions also describe the exact shapes of the inner and outer electrodes, because they must form equipotential surfaces. The embodiments listed also include those where the analyte ions oscillate in a radial direction in a plane between two or more inner electrodes. The analyte ions oscillating radially in this way can then execute harmonic oscillations in the longitudinal direction. The measurement of these harmonic oscillations produces a highly resolved mass spectrum. The advantage of these embodiments with radial oscillations in one plane is that the requirements with respect to the homogeneity of the kinetic energy of the injected analyte ions are very low because ions with both broad and narrow radial oscillations are stored. If the analyte ions are introduced close to the potential minimum of the longitudinal axis potential, they can be collected locally in this minimum for some time before being excited to execute harmonic oscillations in the longitudinal direction.
Mass spectrometers can only ever determine the ratio of the ion mass to the charge of the ion. In the following, the term “mass of an ion” or “ion mass” always refers to the ratio of the mass m to the number of elementary charges z of the ion, i.e., the mass-to-elementary charge ratio m/z. There are several criteria for determining the quality of a mass spectrometer, the main ones being the mass resolution and the mass accuracy. The mass resolution is defined as R=m/Δm, where R is the resolution, m the mass of one ion measured in units of the mass scale, and Δm the width of the mass signal at half maximum measured in the same units. The term mass accuracy relates to both the statistical spread about a measured mean value and the systematic deviation of the measured mean value from the true value of the mass.
The term “metastable” ions used here relates to those ions which are not stable because they have an excess of internal energy that is larger than the binding energy of individual bonds in the molecule, and which decompose into fragments in a period of between about 10 nanoseconds and about 10 milliseconds (or more). This somewhat strange expression stems from the early days of tandem mass spectrometry, when the fragmentation of the ions in straight flight paths between ion-optical deflecting elements such as magnetic and electric fields was studied, and the ions which decomposed within this time frame were called “metastable”. The fragments can be charged and thus represent fragment ions; or they can be neutral.